1. Field of the Invention
The present invention relates to a laser diode, particularly to a laser diode having a wide emission aperture for high-output operation.
2. Description of the Prior Art
In recent years optical communication technology and optical information processing are playing major roles in various fields Digital optical communication using optical fibers has made possible large increases in data communication densities, and optical disks and laser printers have produced a considerable expansion of the range of optical information processing applications.
The progress of optical communications and optical information processing technology owes much to advances made in the laser diodes used as light sources. The small size and high efficiency that are features of laser diodes have brought these devices into widespread use, for example as light sources for compact disk systems, video disk systems and optical communication networks.
In a laser diode the lasing action is generated by the injection of large quantities of carriers into the P-N junction constituting the active layer. Recent advances in semiconductor technology such as MBE (molecular beam epitaxy) and MOCVD (metal-organic chemical vapor deposition) that make it possible to form epitaxial layers as thin as 1 nm or less, have led to the realization of laser diodes that use quantum well active layers less than 20 nm thick, with higher levels of efficiency and lower drive current requirements (see W. T. Tsang in "Semiconductors and Semimetals," vol. 24, pp 397, Ed. R. Dingle, Academic Press, San Diego (1987)).
With MBE and MOCVD making it possible to grow epitaxial layers with extremely good uniformity, in recent years high-output laser diodes have been fabricated that have a wide oscillation region, in the order of 50 to 200 .mu.m. Such lasers can be broadly divided into two types. One type has a single wide stripe whereby, for laser diode applications, it is possible to utilize epitaxial layers with good uniformity to produce laser oscillation across the whole width of the oscillation region, even when the region has a relatively large width, without giving rise to filament oscillation, that is, to the formation of an extremely fine region produced by self-focussing (see C. J. Changhansnain, E. Kapon, and E. Colas, IEEE Journal of Quantum Electronics, vol. 26, page 1713 (1990); P. Gavrilovic, F. N. Timofeev, T. Haw, and J. E. Williams, IEEE Journal of Quantum Electronics, vol. 27, page 1859 (1991)). The other type is the phase-locked array, in which multiple single-stripe laser elements, each having a stable lasing mode, are optically coupled together in a way that enables a single lasing mode to be produced from the differing individual modes of the array. Epitaxial layer formation, with the good uniformity this provides, is of critical importance for realizing a stable array mode. Array lasers, in particular, provide very sharp emission patterns owing to the phase-locked relationship derived from the elements with different stripes (see D. R. Scifres, R. D. Burnham, and W. Streifer, Applied Physics Letters, vol. 41, page 118 (1982); D. Botez and J. C. Connolly, Applied Physics Letters, vol. 43, page 1096 (1983); and L. J. Mawst, D. Botez, T. J. Roth, and G. Peterson, Applied Physics Letters, vol. 55, page 10 (1989)).
The overall relatively phasal uniformity of the light produced by the above high-output power laser diodes makes it advantageous to use them as the light source in optical disk systems, precision laser printers and other such applications requiring the type of very small spot size provided by a beam that is diffraction limited or reduced to near that point. Although MBE and MOCVD has made it possible to form highly uniform epitaxial layers, it is still not easy to achieve high power laser diodes that have a stable oscillation mode and, overall, a wide emission region As described in the above references, it is intrinsically difficult to produce lowest-order mode oscillation in wide single-stripe lasers; instead, the oscillation is usually a mixture of a multiplicity of modes of different orders. With the differences in the threshold gain needed for oscillation being very small from mode to mode, modal instability is readily caused by various changes, as described below.
In a laser array of multiple stripe elements coupled together, in principle there will be as many modes as there are stripes (see J. K. Butler, D. E. Ackley, and D. Botez, Applied Physics Letters, vol. 44, page 293 (1984)). With the small differences in the requisite gain among the modes, there is a general tendency for the gain differential to decrease as the number of stripes and possible oscillation modes increases. Moreover, increasing number of stripes increases the distance between the outermost stripes, which tends to weaken the coupling therebetween and make phase-locking difficult.
Because such conventional laser diodes having a wide emission aperture permit multiple modes each having a threshold gain that is slightly different from that of the other modes, they are sensitive to various changes. For example, increasing the injection current in order to raise the optical output produces a shortage in the supply of carriers that are converted to form the laser beam, resulting in spatial hole burning in the distribution of the carriers. The changes in the index of refraction thus caused produces distortion in the main lasing mode, increasing the mismatch between the spatial distribution of the mode intensity and the carrier distribution, giving rise to lasing of other modes that more closely match the carrier distribution in which the hole burning has occurred. Also, with the higher output there is an increase in heat which produces changes in temperature distribution, gain distribution and distribution of the index of refraction, leading to modal instability.
Moreover, when the laser is actually incorporated into a system and there is feedback of some of the emitted light caused by reflection from the various optical system components, the lasing mode can be changed by the apparent increase in reflectivity or gain produced by the stronger portions of this feedback light. The amount and the position of feedback light change with time owing to the mechanical vibration of the optical components that reflect the light, often resulting over time in fluctuations in light output caused by changes in the lasing mode. In any applications the instability caused by this type of unpredictable time-related change is the most serious problem. In the case of systems using high-output lasers, the amount of this feedback light is often the biggest obstacle to improving the optical system coupling efficiency. Methods of improving the instability of the laser itself include the use of external resonators and the external injection of light, but these increase the complexity, size and cost of the overall system and as such are not always applicable.
Because the single narrow stripe laser diode, which is used mainly as a light source in optical disk systems, is comprised of a single-mode optical waveguide that generally only allows fundamental mode operation, there is no oscillation in higher-order modes so operation is therefore highly stable However, as the optical waveguide has to be very narrow, generally 5 m or less, to realize single-mode operation, optical output power is limited by the degradation that takes place at the light emitting facet owing to the high intensity of the light. Ultimately, therefore, high output power cannot be achieved as long as the width of the light emitting facet cannot be increased.